GB2079519A

GB2079519A - Dielectric fluid composition with high electrical strength

Info

Publication number: GB2079519A
Application number: GB8119594A
Authority: GB
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1980-06-27
Filing date: 1981-06-25
Publication date: 1982-01-20
Also published as: FR2485791A1; CA1131006A; GB2079519B; NO812133L; DE3124576A1; NO156737B; NO156737C; FR2485791B1; SE8104030L; US4296003A; JPH0159685B2; JPS5743305A

Description

1

SPECIFICATION

Dielectric fluid composition with high electrical strength 1 15 GB 2 079 51 9A 1 This invention relates generally to a dielectric fluid composition and, more particularly, pertains 5 to mixtures of atomized dielectric fluids and insulating gases providing high electrical strength.

As a generaly rule, the higher the density of an insulating gas, the higher is its electrical strength. Sulphur hexafluoride (SF.) gas, for example, is about five times denser than air and has a breakdown strength which is about 2.5 times higher, while compressed SF6 has even higher dielectric strength. One problem in compressing a gas to obtain a high electrical strength 10 is that a stronger vessel is needed to contain the gas. Another consideration is the high cost of SF6 when large quantities are required, as in the case with power transmission cables. As set forth in prior art U.S. Patent No. 4,162,227, it is for these reasons that gas mixtures are employed in order that a high strength dielectric gas which may be expensive can be mixed with one of a poorer strength and lower cost to provide a mixture with a dielectric strength somewhere intermediate between the strength values for each of the two mixture components.

Also, in the same U.S. patent it is disclosed that for some gas mixtures, the dielectric strength may be higher than that of either component at a given temperature and pressure of the mixture.

In U.S. Patent No. 2,990,443 a gas-insulated transformer is described in which SF, gas 20 provides the insulation. To remove heat during the transformer operation, a mechanically atomized fluid is introduced into the SF6 gas and circulated throughout the transformer windings and core. It is inferred that the mechanically atomized fluid does not reduce the dielectric strength of the SF, and it is emphasized in the U.S. Patent 2,990,443 that the function of the SF, is to provide electrical insulation.

As can be seen from the discussion of the prior art, it is well known that certain gas mixtures can have a high electrical strength, and that an atomized fluid can be mixed with SF6 without reducing the electrical strength of SF6' It is an object of this invention to provide a gas/atomized fluid-mixture composition of much higher dielectric strength than that of the gas at a given temperature and pressure. Disclosed herein is a preferred range of droplet sizes required in order 30 for the gas/atomized fluid-mixture composition to have a high dielectric strength.

The invention in its broad form consists in a dielectric fluid mixture with a composition comprising:

(a) a first dielectric gas selected from a group consisting of electronegative gases, electropositive gases, and mixtures thereof, and characterized by (b) a second fluid including a liquid atomized 35 into droplets, said liquid being selected from a group consisting of chlorinated liquids, fluorocarbon liquids, and mixtures thereof, and wherein the droplet sizes range below approxi mately 30 microns.

A preferred embodiment teaches a dielectric fluid composition which comprises a mixture of two fluids, one of which is selected from one group consisting of electronegative gases, such as 40 SF, CCI,F,, C2F, CF,CI, and CF, and mixtures thereof; or from another group consisting of electropositive gases, such as N, and CO, and mixtures thereof; or even from mixtures of the two groups. The other component in the mixture is an atomized liquid which may be a chlorinated liquid, such as tetrachloroethylene (C2C'4), or fluorocarbon liquids, such as perfluoro dibutyl ether (C,F,,O), or mixtures thereof.

The advantage of the dielectric m ixtu re-com position of this invention is that the dielectric strength of the atomized dielectric fluid-insulating gas mixture is considerably greater than that of the gas alone. Typically, at one atmosphere pressure, the atomized dieelectric fluid-insulating gas mixture will be twice as strong as either component alone, while at lower pressures near 40 Torr, the mixture will be more than ten times stronger than either component along at 40 Torr. 50 Primarily, because of the discovery that these fluid composition can have high electrical strength, and as atomized droplets can be generated rapidly, a system of this form gives improved dielectric strength during cold start-ups of a vapor-cooled power transformer. Sec ondly, because the dielectric fluid can be atomized acoustically by a suitable choice of power and frequency input to a piezoceramic transducer, a liquid jet spray can be produced, thereby 55 opening up the possibility of replacing the spray system and pump used in the conventional type of vapor-cooled power transformers. Thirdly, an atomized liquid system has good cooling characteristics.

As explained more fully below, it appears that in order for the atomized dielectric fluid insulating gas mixtures to have high electrical strength, the droplets should preferably be in the 60 range of from approximately 0.11t to about 25[t in diameter.

The invention will be more apparent from the following description of a preferred embodiment to be studied in conjunction with the accompanying drawing wherein:

Figure 1 is a vertical sectional view showing an acoustic foutain vaporcooled transformer; Figure 2 is a graph showing the average electrical breakdown strength versus pressure for 65 2 GB2079519A 2 mixtures of acoustically atomized dielectric fluids and insulating gases, and/or gases; Figure 3 is a graph showing the electrical breakdown strength versus temperature for a mixture of acoustic mist (atomized fluid) of C2CI, and SF, at different pressures; and Figure 4 is a graph showing theC2Cl4vapor temperature and breakdown voltages of SF, 5 C2C14vapor and for the mixture of acousticM'St C2Cl4 and SF,.

The dielectric mixture composition disclosed herein may be used for cooling a heat-producing member within a chamber, such as for example, xray equipment, radar, or a transformer, By way of exemplary illustration in Fig. 1 a power transformer is generally indicated at 11 and it comprises a sealed housing 13, electric heat-developing windings such as a winding assembly 15, and a condenser cooler 17. The power transformer 11 also comprises means 19 for applying ultrasonic vibrations. The housing 13 comprises a sealed enclosure providing an internal chamber 21 in which the transformer 15, the condenser cooler 17 and the means 19 are disposed. The housing 13 is comprised of a suitable rigid material such as metal or glass fiber.

The transformer 15 includes a magnetic core and the coil assembly having electrical windings 15 23 which are disposed in inductive relation with a magnetic core 25. For simplicity, the drawings do not show a support structure or electric leads to the windings 23; however, a pair of electric bushings is shown by way of example.

The condenser cooler 17 comprises a plurality of tubes 29 separated by spaces 31 through which ambient gases, such as air circulate whereby cooler 17 acts as a heat exchange. The upper ends of the tubes communicate with the upper portion of the chamber 21 and the lower ends communicate with the lower portion of said chamber, whereby vapor and mist enter the upper ends of the tubes and, upon condensation, drain into the lower portion of the chamber to be recycled as vapor as set forth hereinafter.

The means 19 for applying ultrasonic vibration is disposed at the lower end portion of the 25 housing 13 and is comprised of at least one ultrasonic vibration- producing device or transducer 33. By way of example, a suitable piezoceramic member is PZT-4 which is a product of the Piezoelectric Division of Vernitron Corporation, Bedford, Ohio. The preferred form of the device 33 is a piezoceramic member having a concave or bowl-shaped configuration for focussing ultrasonic vibration onto the surface of a suitable insulating liquid contained in the member. A 30 plurality of, bowl-like devices or bowls, for e.g., six, are located in the lower portion of the housing 13. The devices 33 are spaced from each other and the spaces are occupied by containers 35 which, like the devices 33 are filled with suitable insulating liquid 37. The upper peripheral portions of the bowls 33 and the containers 35 are in liquid- tight contact so that the level of the liquid in the devices and the containers is maintained at a preselected depth. The 35 containers 35, being filled with insulating liquid 37, serve as reservoirs for the devices 33. As the liquid condenses in the cooler 17, it is made to return to the containers 35 where the liquid overflows into the several devices 33 where proper liquid level is maintained for optimum vapor production. The devices 33 are supported above spaces filled with a material having a low acoustic impedance in relation to the liquid, such as air or SF,. Several containers 35 are supported on material 41 such as tetrafluoroethylene (Known under the trade-name Teflon).

The devices 33 are powered by a power supply 42 having a pulse device 43 assocated therewith. A power cable 45 extends from the power supply 42 to the ultrasonic vibration producing devices 33. Ultrasonic vibrations generated by devices 33 are directed and focused by the bowl-like configurations thereof onto the surface of the insulating liquid 37. As a result, 45 the liquid 37 is cavitated and vaporized by the high-frequency soundwaves generated by the piezoceramic material which cause the surface portions of the liquid to be agitated and projected upwardly to form an acoustic fountain 47 of micromist and vapor molecules in the chamber 21 around and above the transformer windings 23 and core 25 as well as onto the surfaces and crevices and openings therein.

The devices 33 have a preferred diameter of about 10 centimeters and their thickness can be selected so that they can operate at a frequency in the range of from about 0.1 to about 5 MHz frequency. The devices are provided with a backing of air or S176 so that maximum acoustic energy is directed toward a focal point 49. An arrangement of devices 33 may include 6 equally spaced bowls operated by a high frequency power supply of about 1 kilowatt. The exact input 55 power varies, and an arrangement of focussing devices as well as operating frequency depends upon other factors such as the liquid used.

A suitable liquid for this purpose, by way of example, is tetrachloroethylene (C2C'4)' The acoustic fountains 47 may operate continuously with operation of the transformer 15, alternatively, depending upon the pumping efficiency, pulsed operation is possible with a high 60 repetitive rate when the transformer is first switched on, lower rates being used later when the core and coils are at normal operating temperatures. To ensure adequate electrical strength of the micromist at the beginning of the operation, the acoustic fountain 47 of mist may be activated perhaps 10 seconds or so before the transformer is energized, by using a timing mechanism. The acoustic fountains 47 project about 1 to 3 meters in height and may be LISPr' 6.r; z _k i:

X 3 1 & 15 1 GB2079519A 3 in conjunction with strategically placed deflectors 51 to ensure adequate coverage of the windings 23 and the core 25.

As the transformer continues to operate, the micromist and vapor fill the inside of the chamber 21. The micromist vaporizes upon contact with the hot surfaces of the core and windings and the vapor then passes across the top of the chamber into the condenser cooler 17; 5 when in contact with the tubes 29, the vapors condense, drain to the bottom of the cooler, and return as liquid to the lower or sump area of the transformer for recycling.

In accordance with this invention, the insulating liquid 37 is a dielectric fluid composition comprising a mixture of two fluids, one of which is selected from one group consisting of electronegative gases such as, SF, CC12F2, C21761 CF3Cl, and CF,, and mixtures thereof; or from 10 another group consisting of electropositive gases, such as, N2 and C02, and mixtures thereof; or even from mixtures of the two groups, The other fluid in the mixture is selected from the group consisting of atomized liquids which may be chlorinated liquids, such as C2Cl4 (tetrachloroethylene), or fluorocarbon liquids, such as, C,F,,O (perfluorodibutyl ether), or mixtures thereof. A mixture of SF6 and C2CI4 comprises a preferred example of the dielectric fluid composition. The 15 use of atomized dielectric fluid-insulated gas as insulation is significant because such mixtures have high electrical strength, and inasmuch as the atomized droplets are generated rapidly, it provides improved dielectric strength during cold start-up of a vapor-cooled power transformer. Moreover, because the dielectric fluid is capable of being atomized acoustically, a liquid jet or spray is easily produced by a suitable choice of power and frequency. As a result it is possible to 20 eliminate the conventional spray system and pump used in the usual type of vapor-cooled power transformers of prior construction. Moreover, an atomized liquid system has superior cooling characteristics.

With regard to the dielectric strength of atomized dielectric fluidinsulating gas mixtures, breakdown voltage data are illustrated in Figs. 2 and 3. In the illustration, the atomized dielectric fluid was tetra ch loroethylene (C2CIJ and the insulating gases used were sulphurhexafluoride (SFJ and air. As the atomization was carried out acoustically, the mixtures were referred to as acoustic mist C2Cl4 plus SF6. Other methods of atomizing the fluid are described within the purview of this invention, so being as the resultant droplet sizes yield a very high electrical breakdown strength in combination with the first gas (which may be SFB). The breakdown voltage curves in Fig. 2 include data for mixtures of acoustic Mist C2CI4 Plus SF61 acoustic Mist C2C14 plus air, and the gases SF6 and air, over a pressure range of about 40 Torr to about 730 Torr. In Fig. 3, breakdown data are plotted at one quarter atmosphere and one atmosphere for acoustic mist C2C14 plus SF6, but over a temperature range of from - 20'C to + 25'C.

At one atmosphere pressure (Fig. 2) the acoustic M'St C20, and SF6 mixture has twice the breakdown strength of SF61 at similar pressure, while at 40 Torr pressure the mixture is ten times as strong as SF, at 40 Torr. The high dielectric strength (Fig. 3) of the acoustic mist C2C11 and SF, mixture is shown to be maintained over the temperature range of from 20'C to + 25'C. Also, the breakdown voltage at 1 mm gap (Fig. 3) in one atmosphere SF6 above C2CII 40 liquid in a closed vessel is about 15 kVpk at - 20'C, as compared with the breakdown voltage in SF, alone at one atmosphere (Fig. 2) which is about 9 kVpk. In effect, experiments show that by saturating the SF, with C2C14 vapor, at one atmosphere, the breakdown strength of SF6 is improved greater than 60%. The breakdown voltage data (Fig. 4) are presented for a 1 mm gap over the pressure range of about 100 Torr to atmospheric pressure (about 730 Torr) for SF, gas, 45 C2Cl4 vapor, and acoustic mist C2Cl4 plus SF,. The C2C14 vapor is electrically stronger than SF,, and acoustic MIS C2C14 Plu's SF, is electrically stronger than C2Cl4 vapor. Specifically, at one atmosphere, C2CI4 vapor is about 60% stronger than SF,, while the acoustic mist CIC14 plus SF6 is about twice as strong as SF6. The electrical breakdown strength tests performed in the context of the invention included 60 Hz breakdown tests wherein the peak kv breakdown values where 50 recorded, as well as impulse tests, using standard 1.5 X 40 impulse waves to record the kv values for full withstand.

The vapor pressure/temperature measurements for C2C14 are also illustrated (Fig. 4) and show that about 80 watts of power are required to heat 700 CC Of C2C14 fluid, to obtain a vapor pressure of about 400 Torr in four hours (101 joules of energy). The breakdown voltage versus 55 pressure curve forC2Cl4can also be calculated from the following formula:

P2 - P1 V2 = V, e-, where P = 730 Torr P 60 It has been known since 1889 (K Natterer, Anal. Phys. Chem. 88,663, 1889), that vapors of carbon tetrachloride (C0J can increase the dielectric strength of air at atmospheric pressure. Moreover, it is known that vapors of tetrachloroethylene (C2C'4) increase the dielectric strength of SF6 at one atmosphere by about 50%. The effects are probably due to the increased density 4 GB 2 079 519A 4 of the "gas" as the vapors mix with it. Moreover, U.S. Patent No. 4,162, 227 discloses that the dielectric strength of mixtures of two or more gases can be higher than that of any of the individual gases at the same temperature and pressure, provided that the strength of one or more of the gases increases at less than one linear rate with increasing pressure.

It has been experimentally found that small quantities of C2C'4 vapor enhance the dielectric strength of SF, gas and the atomizing technique described previously represents a rapid method of introducing vapor into a gas, as set forth herein.

The vapor pressure associated with liquid droplets is higher than the saturated vapor pressure (SVP) above a liquid and its value is calculated from the following equation derived by Lord Kelvin:

1 n p 2Ma P. RTPT where P. is the saturated vapor pressure over a flat surface; P is the saturated vapor pressure at the droplet surface; M is the molecular weight of the droplet; a is the droplet surface tension in dyne-cm; p is the droplet density in gM/CM3; R is the gas constant and T is the absolute temperature in 'K; and 7- is the droplet radius in cm.

Saturated vapor pressure for droplets of water and C20, at 20C, ranging in size from 0.002 20 It to 100 u in diameter, in air, are given in Table 1 below:

TABLE 1

VAPOR PRESSURES OF WATER AND C2C1, DROPLETS IN AIR (25'C) Droplet Diameter (g) 0.002 0.02 0.2 2.0 30 100 Droplet Radius (cm) 10-7 10-6 10-5 10-4 15X 10-4 5 X 10-3 P/P. water droplets 3.16 1.13 1.012 1.001 1.00008 1.000024 in air P/Po C2C14 droplets 13.74 1.30 1.027 1.0026 1.00026 1.000026 in air 30 The saturated vapor pressure in a gas above a liquid, and the saturated vapor pressure of liquid droplets in the gas are factors which determine the rate of evaporation of the droplet and its stability. In order, for say, a 0.2 [t diameter droplet Of C2Cl4 to be stabilized in air, the vapor associated with the droplet must be supersaturated to the extend of 1.027 (Table 1). In other 35 words, at 25'C the SVP for C2Cl4 is about 18 Torr, so for the 0.2 IL droplet to be stable the super saturation would have to be 1.027 times 18 Torr, or about 18.5 Torr. If this condition is not met, the droplet will evaporate. As the 0.2 u droplet would only enhance the SVP by about 3%, and a 30,u droplet would only have about 0.01 % effect, there would be minimal effect on the electrical breakdown of the vapor through supersaturation. However, these vapor pressure considerations explain one function of the acoustic Mist C2C14 in SF, Probably C2CI4 droplets in the 1 to 10 It range, in SF6 gas, will evaporate until the gas is supersaturated and the droplet diameter is stable. It is probable for droplets with a mean diameter of about 5 u, as they fall slowly (0.25 cm/sec), but not for approximately 30 IL droplets which fall at a rate near 2.5 centimeters per second. From the action of an acoustic M'St Of C2CI4 saturating SF, an 45 increase of the breakdown strength of about 50% at 25'C (Fig. 4) is expected. In experiments it was found that after a "shot" of acoustic M'St Of C2Cl4 into SF, gas, and after the droplets have settled out and are back in the main body of liquid, the SF, breakdown strength is improved by about 50%.

The saturation of the SF, with C2Cl4 plus vapor via the acoustic mist partly explains the high 50 electrical strength of the acoustic mist C2Cl4 plus SF6 mixture, but breakdown in the mist plus vapor plus SF6 is higher than breakdown of vapor plus SF6 (Fig. 4). The further increase in strength is believed to be due to electron capture by the droplets.

Approximate measurements of the droplet diameters of acoustic mist Of C2Cl4 were made both with a microscope and by calculation using Stoke's Law and measurements of the velocity of 55, the droplet descent. The C2Cl4 mist droplets which increased the electrical breakdown strength of SF6 ranged in diameter from about 1 to 10 A and average approximately 7 A in diameter.

Mist droplets which did not increase the electrical breakdown strength of SF6 and even perhaps reduce the strength, appear to be greater or equal to 30 A in diameter. The droplet size has a working range of from about 0. 1 p. to about 25 It in diameter and a preferred range of about 1 ft 60 to about 10 [t. The most desirable mists were very dense, and the mist density in droplets/cc has not been measured, but a reasonable estimate is made from the literature and examples are given in Table 11 below:

4 2 GB 2 079 519A 5 TABLE 11

Mean Droplet Droplets Est. Distance Diameter g Per CM3Between Droplets tt Mass /CM3 2.3 ttgm. Rain Cloud: 33 0.63jugm. Dense Sea Mist: 10 10.0 ggm. Acoustic Mist 5 NaCI 1.2 mHz.

2000 1200 1000 2 x 105 180 From Table 11 it is evident thatacoustic mists are extremely dense, but with a distance 15 between droplets about 180 u, the dimensions do not approach the mean free path of electrons which would be a maximum of about 1 p. Although the mist density does not nearly approach the density of the gas molecules, it is believed that with about 2 times 105 droplets per cubic centimeter, there is a high probability of capturing electrons before an electron avalanch can 20 form and lead to an electrical breakdown.

The hypothesis for the observation in experiments conducted is that the high electrical strength of the acoustic Mist C2CI4 + SF6 is due to a combination of the strength of the gasvapor mixture and the capture of electrons by the mist droplets of C2Cl4

Claims

1. A dielectric fluid mixture with a composition comprising:

(a) a first dielectric gas selected from a group consisting of electronegative gases, electropositive gases, and mixtures thereof, and characterized by (b) a second fluid including a liquid atomized into droplets, said liquid being selected from a group consisting of chlorinated liquids, fluorocarbon liquids, and mixtures thereof, and wherein 30 the droplet sizes range below approximately 30 microns.

2. The composition of claim 1 wherein the first gas is SF63. The composition of claim 1 wherein the first gas is CC12F, 4. The composition of claim 1 wherein the first gas is C2F6- 5. The composition of claim 1 wherein the first gas is CF3Cl 6. The composition of claim 1 wherein the first gas is CF, 7. The composition of claim 1 wherein the first gas is N2 8. The composition of claim 1 wherein the first gas is C02' 9. The composition of claim 1 wherein the second fluid is a fluorocarbon.

10.

11.

12.

13.

The composition of claim 11 wherein the second fluid is a chlorinated liquid. The composition of claim 10 wherein the second fluid 'S C2C14' The composition of claim 9 wherein the second fluid is Cj,Cl. The composition of claim 1 wherein the second fluid is a micromist having a droplet size ranging from about 0.1 micron to about 25 microns in diameter.

14. The composition of claim 1 wherein the droplet size is from about 1 tt to about 10 It. 45 15. The composition of claim 1 wherein the second fluid has a density factor of from about c 2 X 101 about 2 X 101 droplets per cc.

16. The composition of claim 1 wherein said fluid mixture has a pressure ranging from about 40 Torr to about 730 Torr.

Printed for Her Majesty's Stationery Office by Burgess Er Son (Abingdon) Ltd-1 982Published at The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.